The term "twins" usually brings to mind two nearly identical people. But that's not always the case. In rare instances, twins have been born with some unusual differences. Here's a look at some extraordinary pairs.
First, take James and Daniel Kelly, recently profiled by the UK's Guardian. Born in 1993 to Alyson and Errol Kelly in southeast London, one of the boys is black and the other is white. James, who is black, looks like his dad, who is of Jamaican heritage. Daniel is white, like his mom. Normally, the genes would combine, producing two mixed-race children. But in this rare case, the genes did not combine.
A geneticist told the newspaper that there is an explanation for this unusual occurrence. It has to do with the father's Jamaican heritage, which includes both European and African ancestries. In about one in 500 sets of twins born to a couple of this genetic mix, "The father will pass on a lot of European DNA to one child and mostly African DNA to the other. The result will be one white child and one black."
One more fun fact: The siblings, now 18, are the third set of twins for the family. Both the mother and the father had a set of twins with their previous partners. The couple also have a daughter, who is not a twin.
Another surprise was reported back in 2009, after Mia Washington's twins, Justin and Jordan, were born in Texas. The two looked so different that it prompted her fiancé, James Harrison, to give the boys paternity tests. Turns out, the two had the same mom but different dads. Possible? Yes, and it may be more common than you'd think.
Dr. Hilda Hutcherson, a clinical obstetrician, explained on the "Today" show that some women can release more than one egg a month. And if that woman has sexual relations with more than one man when she's ovulating, it's possible for the eggs to be fertilized by different sperm. Hutcherson also said that while it's rare -- maybe one in a million -- studies have suggested that a very small percentage of fraternal twins actually have different fathers.
And here's one for the record books: Hussain Bisad, who is 7 feet 7.5 inches tall, is one of the tallest men in the world -- and has the biggest hand span of any living man, according to Guinness World Records. He also has a twin, Khardra. But it must be hard for the two to have a conversation: She's only about 5 feet 5 inches tall, which prompted Guinness to wonder whether the gaping height difference between the two broke any records for twins.
There is an explanation for the towering twin's stature: Hussain suffers from pituitary gigantism, a tumor that causes excessive growth and that can lead to other health problems.